Conventional power supply apparatus use a switching or series regulator on a power supply side for outputting a predetermined voltage required by a load side. As shown in FIG. 1, a background power supply apparatus 100 has a feedback configuration to return an output voltage Vo of a DC-to-DC converter, to divide the output voltage Vo with a voltage divider 101 into a divided voltage Vd, and to compare the divided voltage Vd with a predetermined reference voltage Vr.
Due to a recent trend, however, the voltage required by the load side has been reduced, and the power supply apparatus is consequently required to change the output voltage of the switching or series regulator. Also, the power supply apparatus itself needs to be changed when a required power supply voltage value is changed by, for example, the replacement of a component used in the load side of a system after the power supply apparatus is installed into the system.
In view of the foregoing, it is desirable to change a level of an output constant voltage in response to a control signal sent from a load side.